1. Field
This invention concerns a highly simplified speed control system for use with electrical motors of all types and phase wiring, especially for blower or fan induction motors used in conjunction with air moving, e.g., air conditioning systems, or with the motors per se which drive refrigerant compressors, whereby the motor can be readily shifted between a full load first speed and a reduced load second speed. Such compressors are shown for example in U.S. Pat. Nos. 4,396,359; 5,129,792; and 5,201,640, the disclosures of which are hereby incorporated herein by reference.
In regard to compressors, vacuum or other pumps or machines, and particularly those reciprocating piston compressors used in single or multiple cylinder refrigeration, air conditioning systems or heat pumps or the like, including machines such as scotch yoke compressors of U.S. Pat. No. 4,838,769, it is often desirable to vary the compressor output, i.e., compressor capacity modulation, in accordance with cooling load requirements. Such modulation allows large gains in efficiency while normally providing reduced sound, improved reliability, and improved creature comfort including one or more of reduced air noise, better de-humidification, warmer air in heat pump mode, or the like.
With reference to the blower or fan motor used with such modulatable compressors, the need for a motor speed control system which reduces the input power to the motor in proportion to the power necessary to attain the desired optimum CFM for the reduced capacity mode arises, e.g., from the fact that when the compressor capacity is reduced approximately 50%, if the evaporator blower motor is maintained at high CFM air output, the capacity of the evaporator coil is reached at some airflow level near 50% of full compressor capacity. As the function of the evaporator is to change liquid to vapor and thus absorb host, if the evaporator capacity is reached, the heat absorbed becomes sensible and further heat is absorbed by the refrigerant itself which reduces Rs molecular density and the overall efficiency of the system is thus compromized since the power input to the blower should be reduced by the cube root of the full speed blower motor power.
Such reduction in motor speed however, is not without its difficulties, as many negative electrical phenomena are associated with conventional motor speed reducers which employ such means as winding tapping, solid state voltage reduction, i.e., wave form chopping to change induction motor speed in response to a change in cooling load requirements, whether it be with respect to the blower motor or the compressor motor. One such phenomenon is the undesirable reduction in motor efficiency, i.e., Power out/Power in ratio, which occurs when the motor "designed optimum load point" is underreached, This phenomenon is classically represented by the speed/torque/efficiency curve well known to the electrical motor art.
2. Prior Art
Heretofore, the only readily available equipment on the market for providing full range optimum efficiency is the totally variable speed, motor controller or inverter devices such as that shown schematically on page 9 and described fully in the 63 page .RTM.1996 Warner Electric, .RTM.SECO Electronics installation & Operation Manual for SECO.RTM.AC Drive, SL 3000 Series AC Motor Drives, the disclosures of which Manual is hereby incorporated herein by reference in its entirety.
Further details and theory of electrical circuitry for inverters in general is given on pages 440-461 of the book entitled ELECTRICITY, Principles and Applications, by Richard Fowler, Western Washington University, McGraw-Hill Book Company, Gregg Division, ISBN-0-07-021704-1, which publication is hereby incorporated herein in its enurety.
However, such inverters are expensive and complex since they must give wide band operation. Such inverters ore typically sized for the highest f/speed, e.g., max motor speed and max Hz. Such sizing requires large, heavy electric load carrying components, more complex micro processors, heavier DC current and the like. Also, their complexltes adversely affect their reliability, particularly as the result of lengthy use for varying motor speed over a wide range.
While such inverters find use in the present invention, it is preferred to use inverters which are properly sized to handle the reduced voltages and currents which attend the reduced motor operation "at between about 20 to 50 Hz, and, more preferably at between about 32 to 37 HZ" after operation.